The following description of the background of the invention is provided to aid in understanding the invention, but is not admitted to describe or constitute prior art to the invention.
Cellular signal transduction is a fundamental mechanism whereby extracellular stimuli are relayed to the interior of cells and subsequently regulate diverse cellular processes. One of the key biochemical mechanisms of signal transduction involves the reversible phosphorylation of proteins. Phosphorylation of polypeptides regulates the activity of mature proteins by altering their structure and function. Phosphate most often resides on the hydroxyl moiety (--OH) of serine, threonine, or tyrosine amino acids in proteins.
Enzymes that mediate phosphorylation of cellular effectors generally fall into two classes. The first class consists of protein kinases which transfer a phosphate moiety from adenosine triphosphate to protein substrates. The second class consists of protein phosphatases which hydrolyze phosphate moieties from phosphoryl protein substrates. The converse functions of protein kinases and protein phosphatases balance and regulate the flow of signals in signal transduction processes.
Protein kinases and protein phosphatases are generally divided into two groups--receptor and non-receptor type proteins. Most receptor-type protein tyrosine phosphatases contain two conserved catalytic domains, each of which encompasses a segment of 240 amino acid residues. Saito et al., 1991, Cell Growth and Diff. 2:59-65. Receptor protein tyrosine phosphatases can be subclassified further based upon the amino acid sequence diversity of their extracellular domains. Saito et al., supra; Krueger et al., 1992, Proc. Natl. Acad. Sci. USA 89:7417-7421.
Protein kinases and protein phosphatases are also typically divided into three classes based upon the amino acids they act upon. Some catalyze the addition or hydrolysis of phosphate on serine or threonine only, some catalyze the addition or hydrolysis of phosphate on tyrosine only, and some catalyze the addition or hydrolysis of phosphate on serine, threonine, and tyrosine.
Tyrosine phosphatases can down-regulate the catalytic activity of protein kinases involved in cell proliferation and are therefore thought to be possible anti-cancer proteins. Protein phosphatases with inappropriate activity are also involved in some types of cancer. Because abnormally elevated levels of cell proliferation are associated with receptor and non-receptor protein kinases with unregulated activity, protein phosphatase-catalyzed dephosphorylation of a protein kinase can down-regulate kinase activity and thereby decrease the rate of cell proliferation.
In addition to their role in cellular proliferation, protein phosphatases are thought to be involved in cellular differentiation processes. Cell differentiation occurs in some cells upon nerve growth factor (NGF) or epidermal growth factor (EGF) stimulation. Cellular differentiation is characterized by rapid membrane ruffling, cell flattening, and increases in cell adhesion. Chao, 1992, Cell 68:995-997.
Alignment of primary amino acid sequences of known PTPs shows that their catalytic domains share common amino acid sequences. This observation has facilitated efforts of cloning protein phosphatases from multiple organisms and tissues. Probing cDNA libraries with polynucleotides complementary to cDNA encoding protein phosphatase consensus sequences has identified cDNAs resembling protein phosphatase sequences via the polymerase chain reaction (PCR). Some polypeptide molecules encoded by these cDNAs have tyrosine phosphatase activity.